ORGANIC LETTERS
Steady-State and Time-Resolved Studies on Oxetane Cycloreversion Using (Thia)pyrylium Salts as Electron-Transfer Photosensitizers
2001 Vol. 3, No. 13 1965-1967
Miguel A. Miranda,* M. Angeles Izquierdo, and Francisco Galindo Departamento de Quı´mica/Instituto de Tecnologı´a Quı´mica UPV-CSIC, UniVersidad Polite´ cnica de Valencia, Camino Vera s/n, Apdo. 22012, 46022 Valencia, Spain
[email protected] Received March 15, 2001
ABSTRACT
Cycloreversion of oxetane 1 is achieved using (thia)pyrylium salts as photosensitizers. RadicaI cation intermediates involved in the electrontransfer process have been detected using laser flash photolysis. The experimental results are consistent with the reaction taking place from the triplet excited state of the sensitizer.
Cycloreversion (CR) of oxetanes by photoinduced electron transfer (PET) has recently attracted considerable interest, as this process appears to be involved in the photoenzymatic repair of the (6-4) photoproducts of the DNA dipyrimidine sites by photolyase.1 Both radical anions and radical cations of oxetanes undergo fragmentation, though reaction with the former is more exothermic.2 In a preliminary study using 2,2-diaryloxetanes and cyanoaromatic compounds as electron-transfer photosensitizers, the CR reaction was found to occur through the excited singlet state of the photosensitizer.3 Although the process was explained via formation of radical cations, no intermediate of this type was detected. In addition, cycloreversion occurred with cleavage of the same C-C bonds formed in the Paterno-Bu¨chi photocycloaddition employed to synthesize the starting 2,2-diaryloxetanes. A radical chain mech(1) Prakash, G.; Falvey D. E. J. Am. Chem. Soc. 1995, 117, 1137511376. (2) Wang, Y.; Gaspar, P. P.; Taylor, J. S. J. Am. Chem. Soc. 2000, 122, 5510-5519. (3) Nakabayashi, K.; Kojima, J. I.; Tanabe, K.; Yasuda, M.; Shima, K. Bull. Chem. Soc. Jpn. 1989, 62, 96-101. 10.1021/ol0158516 CCC: $20.00 Published on Web 05/31/2001
© 2001 American Chemical Society
anism was found to operate when electron-donating substituents are attached to the aryl groups.3 Pyrylium salts are a potentially useful group of photosensitizers.4 They are good oxidizing agents and can be selectively excited because of their absorption in the visible.5 On the other hand, depending on the substitution pattern and the reaction conditions, they can generate radical ion pairs of different multiplicity (singlet or triplet).6,7 Thus it may be of interest to study the influence of this factor on the reaction, for instance, regarding the splitting mode and the stereoselectivity of the products. In the present work, we have chosen the diphenyl-substituted oxetane 18 and a series of pyrylium and thiapyrylium salts 2a-d as model compounds (4) Miranda, M. A.; Garcı´a, H. Chem. ReV. 1994, 94, 1063-1089. (5) Martiny, M.; Steckhan, E.; Esch, T. Chem. Ber. 1993, 126, 16711682. (6) Akaba, R.; Arai, S. J. Chem. Soc. Chem. Commun. 1987, 12621263. (7) (a) Akaba, R.; Sakuragi, H.; Tokumaru, K. Chem. Phys. Lett. 1990, 174, 80-84. (b) Akaba, R.; Oshima, K.; Kawai, Y.; Obuchi, Y.; Negishi, A.; Sakuragi, H.; Tokumaru, K. Tetrahedron Lett. 1991, 32, 109-112. (c) Kuriyama, Y.; Arai, T.; Sakuragi, H.; Tokumaru, K. Chem. Lett. 1988, 1193-1196. (d) Akaba, R.; Kamata, M.; Koike, A.; Mogi, K. E.; Kuriyama, Y.; Sakuragi, H. J. Phys. Org. Chem. 1997, 10, 861-869. (8) Fleming, S. A.; Gao, J. J. Tetrahedron Lett. 1997, 38, 5407-5410.
to study the photosensitized electron-transfer cycloreversion of oxetanes and to gain further insight into the mechanistic aspects of this reaction.
nometer.9 The results are also given in Table 1. These results clearly show that (a) the CR is photosensitized by pyrylium salts, (b) it produces carbonyl and olefin units different from the reagents employed in the Paterno-Bu¨chi process, and (c) the most efficient photosensitizer is the thiapyrylium salt 2b. Moreover, the fact that the quantum yields were much less than unity suggest that a chain reaction is not involved. To elucidate the reaction mechanism actually operating, a laser flash photolysis (LFP) study was performed. In a typical experiment, LFP (355 nm) of a mixture of 1 and 2b in CH3CN gave a transient with an absorption maximum at 470 nm (Figure 1), which can be ascribed to the radical cation of trans-stilbene on the basis of literature data.7,10
When oxetane 1 was irradiated (λ > 340 nm) using catalytic amounts of pyrylium salts 2a-d, CR was observed, resulting in the production of trans-stilbene and acetaldehyde as the only photoproducts (Scheme 1).
Scheme 1
Figure 1. Transient spectra obtained from LFP (λ ) 355 nm) of 1 (1.2 × 10-4 M) and 2b (10-4 M) in acetonitrile, under argon.
The reaction yields are given in Table 1. Control experiments showed that no photocycloreversion takes place in the dark or in the absence of the photosensitizer. Further control
Table 1. Cycloreversion of Oxetane 1 Photosensitized by Pyrylium Salts 2a-d sensitizer
CR (%)a
CR (φ)b,c
2a 2b 2c 2d
8d 100 33d
0.02 0.10 0.04
Oxetane 1, 4.7 × 10-2 M; sensitizer, 10-3 M; solvent, CDCl3; filter, λ > 340 nm; Luzchem multilamp photoreactor, 8 W lamps (4×) with emission maximum at 300 nm. b Oxetane 1, 10-4 M; sensitizer, 2.5 × 10-5 M; solvent, CH3CN; lamp, Xenon 450 W, monochromatic light of λ ) 420 nm; actinometer, potassium ferrioxalate. The solutions were placed in sealed cuvettes and bubbled with argon for 15 min to achieve deoxygenation.c No significant CR reaction was observed in the presence of oxygen. d The rest was unreacted starting material.
This confirmed that photocycloreversion of oxetane 1 follows an electron-transfer mechanism and indicated that splitting of the four-membered ring gives rise to the radical cation of trans-stilbene instead of the radical cation of 1-phenylpropene. In principle the PET process between 1 and 2 might involve either the excited singlet or triplet of the pyrylium salt. To check the feasibility of the two pathways, the free energy change associated with electron transfer was calculated using the Weller11 equation (eq 1) for both excited states. ∆G (kcal/mol) ) 23.06 × [E(D+•/D) - E(A/A-•)] - E*A (1)
a
experiments (data not given in Table 1) clearly showed that neither the photosensitizer concentration (up to 10-2 M) nor the nature of the counteranion (BF4- or ClO4-) produced significant effects on the obtained results. To obtain more reliable quantitative data, the CR quantum yield was measured using potassium ferrioxalate as acti1966
The ED+•/D for 1 was measured by cyclic voltammetry in acetonitrile and found to be 1.42 V vs SCE. On the other hand, the singlet and triplet energies of the sensitizers had been previously measured.4 Using these values, the PET reaction was found to be exothermic in all cases (Table 2). (9) Hatchard, C. G.; Parker, C. A. Proc. R. Soc. London, Ser. A 1956, 235, 518-536. Braun, A. M.; Maurette, M. T.; Oliveros, E. Technologie Photochimique; Presses Polytechniques Romandes: Lausanne, 1986; p 62. (10) (a) Shida, T. Electronic Absortion Spectra of Radical Ions; Elsevier: Amsterdam, 1988; p 113. (b) Hamil, W. H. Radical Ions; Kaiser, E. T., Kevan, L., Ed.; Interscience: New York, 1968; p 399. (c) Spada, L. T.; Lewis, F. D.; Bedell, A. M.; Dykstra, R. E.; Elbert, J. E.; Gould, I. R.; Farid, S. J. Am. Chem. Soc. 1990, 112, 8055-8064. (11) Rehm, D.; Weller, A. Isr. J. Chem. 1970, 8, 259-271. Org. Lett., Vol. 3, No. 13, 2001
Scheme 2
Table 2. Thermodynamics of the PET Reaction between 1 and 2a-d
d
sensitizer
E(A/A-•.)a
E(S)b,c
E(T)b,c
∆G(S)b
∆G(T)b
2a 2b 2c 2d
2.53 2.59 2.49 1.98
66 66 63 58
53 52 nad 51
-24 -26 -23 -11
-11 -12 nad -4
a Given in V (vs SCE). b Given in kcal/mol. c Taken from the literature.4 na: not available.
To gain further insight into the reaction mechanism, quenching of the pyrylium salts fluorescence by the oxetane was investigated. Pyrylium salt concentration was fixed by adjusting the absorbance of the solutions at the arbitrary value of 0.19. Then, the fluorescence intensities (I) observed upon addition of different concentrations of 1 ([Q]) were used to calculate the Stern-Volmer constants (K). A linear fit was observed in all cases according to the equation I0/I ) 1 + K[Q]. These values together with the fluorescence lifetimes (measured by time-resolved emission spectroscopy) were used to calculate the quenching rate constants (kq). The results are shown in Table 3. From these data it is clear that fluorescence quenching occurs at a nearly difusion-controlled rate, except in the case of the methoxy derivative 2d (Table 3).
the different singlet deactivation pathways under the employed reaction conditions (Table 4).
Table 4. Relative Contribution of the Different Pathways Resulting in Deactivation of the Excited Singlet of 2a-d at Two Concentrations of 1 [1] (M)
sensitizer
Q (%)a
F (%)b
ISC (%)c
10-2
2a 2b 2c 2d 2a 2b 2c 2d
23 23 12
36 5 29 97 46 6 33 97
41 72 59 3 53 93 66 3
10-4
a
Table 3. Deactivation (Fluorescence and Intersystem Crossing) of the Excited Singlet of 2a-d sensitizer
10-8 × kfa,b
10-8 × kisca,b
φiscb
10-9 × kqc
2a 2b 2c 2d
1.1 0.1 2.5 1.8
1.2 2.1 5.1
